Conventionally, a sealed lithium ion secondary battery to be mounted on a vehicle or other equipment may contain gas in it due to overcharge, excessive temperature rise, breakage by an external force, or the like, the gas leading to a higher pressure inside the battery. Therefore, a weakened location for safety is formed in a battery case of such a sealed battery.
For example, it has been proposed that a member deformable by an internal pressure of the battery is used in a part of the battery case, deformation of which member breaks the weakened part to break a current path or makes communication between inside and outside of the sealed battery to discharge the gas.
For example, Patent Literature 1 discloses a technology of including a sealing body lead and a diaphragm between an external terminal and an internal collector tab and sealing a periphery of the diaphragm in the cylindrical sealing body lead extending orthogonally to a battery lid, in which, as the battery internal pressure rises, the diaphragm deforms to rupture the weakened location, thereby breaking the path of the current.